1. Field of the Invention
The invention relates to a computed tomography method which includes the following steps: generating, while using a radiation source, a conical radiation beam which traverses an examination zone or an object present therein, generating a circular relative motion, including a rotation about an axis of rotation, between the radiation source on the one side and the examination zone or the object on the other side, acquiring, while using a detector unit, measuring data which is dependent on the intensity in the radiation beam to the other side of the examination zone during the relative motion, reconstructing the spatial distribution of the absorption within the examination zone from the acquired measuring data. The invention also relates to a computed tomography apparatus for carrying out the above method.
2. Description of Related Art
A method of the kind set forth is known from a publication by L. A. Feldkamp et al. "Practical cone-beam algorithm", Journal of Optical Soc. Am. A/Vol. 1, No. 6, 1984, pp. 612-619. The known method in principle consists of the following steps:
a) multiplying, for all radiation source positions, all measuring values by a weighting factor which corresponds to the cosine of the angle enclosed by the ray along which the measuring value has been acquired with respect to the central ray; PA0 b) subjecting the measured values thus weighted to a high-pass filtering operation. PA0 c) backprojecting the measured values into the examination zone along the rays along which they have been measured. The contribution of a measuring value to the absorption value of a voxel must then be weighted by a factor which is dependent on the distance between the relevant voxel and the radiation source position. PA0 a) rebinning the measuring data so as to form a number of groups, each group containing a plurality of planes which extend parallel to one another and parallel to the axis of rotation and in each of which a respective fan beam is situated, PA0 b) one-dimensional filtering of the data produced by the rebinning in each group in a direction perpendicular to the direction of the planes, PA0 c) reconstructing the spatial distribution of the absorption by backprojection of the filtered data of a plurality of groups.
Because this last step must be carried out for all voxels of the volume to be reconstructed and for all radiation source positions, it requires a long calculation time. It is a further drawback of the known method that it only enables the reconstruction of the absorption in voxels which have been continuously exposed to X-rays during the examination. Such voxels are situated in a discus-shaped region which is concentrically situated relative to the axis of rotation. However, it is desirable to perform a reconstruction in a flat, cylindrical slab-like region. If the reconstruction, however, is limited to the slab-like region in the conical beam which is exposed to X-rays in all radiation source positions, only a very narrow reconstruction region will be obtained.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.